Motor-driven hydraulic power steering apparatuses are conventionally known which are adapted to rotate an oil pump by an electric motor and supply a working oil to a power cylinder from the oil pump to mitigate an operation force required for operating a steering wheel.
In a power steering apparatus of this type, a steering assist force is determined by the rotation speed of the electric motor. More specifically, a higher rotation speed of the electric motor increases the flow rate of the oil to increase the assist force, while a lower rotation speed of the electric motor reduces the flow rate of the oil to reduce the assist force.
On the other hand, the assist force is not required when the steering operation is not performed and, therefore, the rotation of the electric motor is stopped or kept in an idling state with an extremely low rotation speed for power saving.
Since loads on front wheels are apparently increased immediately after a vehicle is decelerated by pedaling a break just before a corner, the steering wheel tends to feel heavier. Immediately after the vehicle is accelerated, the loads on the front wheels are apparently reduced, so that the steering wheel tends to feel lighter. With the assist force being maintained at a constant level, the steering feeling changes at the acceleration and deceleration of the vehicle, so that the driver feels uncomfortable.
When the steering operation is suddenly started in a non-steering state, the rotation of the electric motor cannot follow the sudden start of the steering operation, so that the steering wheel temporarily feels heavier. Therefore, it is desirable that the rotation speed of the electric motor is steeply increased by predicting occurrence of the steering operation.